Japan should be prepared for the possibility of a magnitude-10 earthquake, although the chances of a temblor that size are slim, a seismologist said.

"The chances of a magnitude-10 occurring are very low," professor Toru Matsuzawa of Tohoku University reported at a Nov. 21 meeting of the Coordinating Committee for Earthquake Prediction, Japan. "But if we think of what could happen, with the maximum in mind, we can make a swift response."

The world's largest recorded earthquake was the magnitude-9.5 Valdivia earthquake off the coast of Chile in 1960, rupturing a 1,000-kilometer fault. A magnitude-10 earthquake would be 30 times more powerful than the magnitude-9.0 Great East Japan Earthquake of 2011.

Matsuzawa said a magnitude-10 quake is possible in theory if a large fault slips.

If a temblor of such a scale should strike, the underground rupture would continue for 20 minutes to an hour, meaning tsunami could hit coasts before the shaking subsides.

If a 3,000-kilometer stretch from the Japan Trench to the Kuril-Kamchatka Trench along the Pacific Ring of Fire has a slip of 60 meters, that would constitute a magnitude-10 earthquake, he said.

The energy of a magnitude-11 quake, 30 times more powerful than a magnitude-10 temblor, would be equivalent to that of the asteroid impact that is believed to have caused the extinction of the dinosaurs 65.5 million years ago. Such a quake would cause a shift extending more than 20,000 kilometers. Matsuzawa concluded that consideration would not be needed for such an earthquake.